The latest research out of Stanford University suggest that ant
infestations are the result of weather, especially cold and wet
conditions, and unusually hot and dry spells (probably bec they're
trying to find shelter from extreme weather). According to the
researchers, insecticides are also ineffective at controlling the
infestations. The best thing to do is to close up entrances the ants
are using, and wipe down the ant trail with Windex to eliminate their
scent trail. Read about this at
http://www.stanford.edu/dept/news/pr/01/ants45.html